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Leon[_7_] Leon[_7_] is offline
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Default Wide shelving advice needed

On 2/13/2020 1:12 PM, dpb wrote:
On 2/13/2020 9:36 AM, Leon wrote:
On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal
wrote:
whit3rd writes:

Â* Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes.Â*Â* Stronger in the shelf-sag
sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile
strength
in the bottom surface (knots on top are less troublesome than on
bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good
high-tensile strength wood,
Â* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".Â* In fact there
is more "in the right direction" plies then the wrong.Â* If that
means anything.
...

Only that it's somewhat more rigid in the direction of the
longitudinal plies than the other...but it's still less than solid
wood longitudinal of the same species.

Â*From US FPL Handbook Chap 12 on mechanical properties a summary
table shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq
whereas plywood is 1.01-1.24 or only about half.Â* Since difference in
deflection of two pieces of same size and species is only the effect
of the different E as the geometrical factor is the same, the
computed sag is directly proportional to the inverse of the E.Â* IOW,
the sag for those two is almost 2X for ply vis a vis solid of same
dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one
of the firs it gives specifically, they all were less (altho not by
factor of 2 which does seem somewhat excessive by common experience).
It doesn't have any other plywood to compare against.


--



The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions.Â* What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow
less along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood?Â* You add support.Â* Typically along the front edge and
often along the back edge.Â* Lets say you use equal steel beams to
reinforce the length of the shelves so that there is no bow at all
along the front and back edge

Add weight to the center until one fails.Â* My money is on the plywood
coming out the winner.Â* The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length?Â* Yes,
especially as it get older.Â* And that was not necessarily with a lot
of weight. And the deeper the shelving the more likely the solid wood
will fail over plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.


Well, the poster of the above comment was used the word strength in an
imprecise manner and really was referring to the ability to resist
bending moment which is, for a fixed geometry, directly proportional to
the modulus of elasticity.

What you're describing also is not really "strength" per se, but the
effect of the very high anisotropic nature of wood with and across
grain.


Actually the addition of adhesives between ply's, a necessary component,
makes plywood stronger.